The increasing use of mobile voice and data communications requires a more efficient utilisation of available radio-frequency resources. In order to increase data transmission performance and reliability, the so-called multiple input and multiple output (MIMO) technology may be used in wireless radio telecommunication systems for transmitting information between a base station and a user equipment, for example mobile devices like mobile phones, mobile computers, tablet computers, wearable devices and stationary devices like personal computers or cash registers. MIMO systems may use multiple send and receive antennas at the base station as well as at the user equipment. The MIMO technology forms the basis for coding techniques which use the temporal as well as the spatial dimension for transmitting information. The enhanced coding provided in MIMO or systems allows the spectral and energy efficiency of the wireless communication to be increased.
The spatial dimension may be used by spatial multiplexing. The spatial multiplexing is a transmission technique in MIMO wireless communications to transmit independent and separately encoded data signals, so-called streams, from each of the multiple transmit antennas or a combination thereof. Therefore, the space dimension is reused or multiplexed more than one time.
If the transmitter is equipped with NT antennas and the receiver has NR antennas, the maximum spatial multiplexing order NS (the number of streams or the rank) is NS=min (NT, NR). This means that NS streams can be transmitted in parallel, ideally leading to an NS increase of the spectral efficiency (the number of bits per second and per Hz that that can be transmitted over the wireless channel). For example, a MIMO system with a base station having two antennas and a user equipment having two antennas may operate at a rank of up to 2 and is also called 2×2 MIMO, indicating the number of antennas at the base station and at the user equipment.
The so-called full dimensional MIMO (FDMIMO) refers to a technology that arranges the signals transmitted to antennas in the form of beams that are able to power multiple receivers in three dimensions. For example, a base station may comprise a large number of active antenna elements in a two-dimensional grid and the FDMIMO technology is capable of supporting many users on the same time/frequency resource blocks simultaneously. This reduces interference from overlapping transmissions to other receivers and increases the power of the signal. The beams may form virtual sectors which may be static or dynamic in view of the base station. The large number of antennas of the base station allows radio energy to be spatially focused in transmissions as well as a directional sensitive reception which improves spectral efficiency and radiated energy efficiency. In order to adapt the transmit signal at each individual antenna of the base station in accordance with the currently active receiving user equipment, a base station logic needs information about radio channel properties between the user equipment and the antennas of the base station. Vice versa, in order to adapt the transmit signal at each individual antenna of the user equipment, a user equipment logic needs information about the radio channel properties between the base station and the antennas of the user equipment.
For this purpose, a so-called channel sounding may be performed to determine the radio channel properties between the user equipment and the base station. For example, a pilot signalling scheme can be used for this purpose which allows the base station to set configuration antenna parameters for transmitting signals, so as to focus radio energy at the user equipment, or for receiving radio signals from the user equipment. Likewise, the pilot signalling scheme can be used to enable the user equipment to set antenna configuration parameters for transmitting signals, so as to focus radio energy at the base station, or for receiving radio signals from the base station.
In a sectorised full dimension MIMO, in each sector the base station may perform such a channel sounding. However, when the operational frequency increases and consequently the wavelength decreases, the antenna aperture becomes small and therefore multiple antennas may be utilised at the receiver to increase the received power. In particular in case of high transmission frequencies of for example 30 GHz or more and multiple antennas having small apertures, the reception sensitivity of the user equipment may significantly depend on polarisation of the transmitted radio-frequency signals. The channel sounding reveals only information about the radio-frequency channel characteristics between the user equipment in its current orientation and the base station.
In view of the above, there is a need in the art for methods and devices which address at least some of the above shortcomings of conventional MIMO systems. In particular, there is a need in the art for improving operation of a wireless communication system to reduce the power losses of wireless communications due to polarization misalignments.